A more complete version of the Racker reference quoted above by Losada (1978):
However, pyruvate has only 4 hydrogens to donate to 2 oxygens and with a P:O ratio of 3 should therefore yield only 6 molecules of ATP (Table 1.2). Where are the other 6 hydrogens coming from so that 15 ATP can be formed? The answer to this question gives us what I think is the key to the puzzle why nature has designed the complex acrobatic scheme of the Krebs cycle. Its major purpose I believe is to increase the energy yield by catalyzing cleavage of water.
There are three steps at which water enter the Krebs cycle. - one at the transformation of fumarate to malate an the other two somewhat more indirectly during the utilization of acetyl-CoA and succinyl-CoA. In the course of the Krebs cycle the hydrogens of these water molecules are separated from oxygen and are donated to DPN or to a flavoprotein (e.g., succinate dehydrogenase) and then transported via the oxidation chain of mitochondria as electrons and protons as we shall discuss later.
[Racker, E. (1976). A New Look at Mechanisms of Bioenergetics, pp 5-6, Academic Press].
IMO, the last sentence of this quote is complete rubbish. (For DPN, read NAD. DPN, or diphospo-pyridine-nucleotide (or something close), is an old name for NAD)
